This invention was developed at least in part under U.S. Air Force Contract F19628-91-C-0082, and the U.S. government has certain rights in this invention pursuant to this contract.
The present invention relates generally to a superconductor material. More particularly, the invention relates to a textured, high temperature superconductor ceramic ("HTSC") material and a method of producing this material at low temperatures.
High temperature superconducting ceramics (HTSC) are intrinsically weak and brittle materials. In addition, conventional ceramic processing of these materials produces polycrystalline bodies which have low critical current densities (j.sub.c, in DC measurements) and high surface resistivities (R.sub.S, in RF measurements). Commercial applications of these materials require components that exhibit high j.sub.c and/or low R.sub.s values as well as the capability of producing mechanically strong and easy to manufacture components. Due to the materials' low mechanical strength, commercially useful structures cannot be produced without the use of a substrate to impart strength and toughness to the superconductor. This is especially true for lower frequency RF devices that will require the superconductor to be formed into relatively large, complex shapes. HTSC thin films have been shown to have high current densities and low R.sub.s values. However, these films are not useful for low frequency RF applications because they require expensive single crystal substrates (typically, LaAlO.sub.4 or SrTiO.sub.4) and can only be formed into planar structures with dimensions under a few inches.
Bulk HTSC materials with highly textured microstructures can exhibit the level of electrical performance required for commercial applications. For YBa.sub.2 Cu.sub.3 O.sub.7-x, such textured microstructures are produced using a method called peritectic recrystallization or, more commonly, "melt-texturing". In this process, "textured" YBa.sub.2 Cu.sub.3 O.sub.7-x is produced by crystallizing this compound out of its peritectic mixture of Y.sub.2 BaCuO.sub.5 plus a Ba/Cu-rich liquid. Many variations of this technique have been described, and it is commonly practiced in laboratories throughout the world. However, the process remains essentially the same as that originally developed in 1988.
The melt-tearing process typically involves heating a sample above the peritectic temperature (1015.degree. C. in air) to decompose the YBa.sub.2 Cu.sub.3 O.sub.7-x into Y.sub.2 BaCuO.sub.5 plus liquid. This mixture is cooled slowly through the peritectic temperature allowing YBa.sub.2 Cu.sub.3 O.sub.7-x to crystallize. When this cooling is performed in the presence of a thermal gradient, the YBa.sub.2 Cu.sub.3 O.sub.7-x grains preferentially grow parallel to the gradient and a "teared" microstructure results. The slow cooling keeps the nucleation rate of YBa.sub.2 Cu.sub.3 O.sub.7-x low, resulting in the formation of a small number of nuclei. As a result, the YBa.sub.2 Cu.sub.3 O.sub.7-x grains can grow to very large sizes before impingement; and if the cooling is performed in a thermal gradient, the grains will be highly aligned. In the originally developed process samples were measured to have critical currents of up to 17,000 A/cm.sup.2 in self-field with only a small magnetic field dependence. Improvements to this process (which have included the production of continuous lengths of melt-textured filaments) have resulted in measured current densities as high as 140,000 A/cm.sup.2 in self field and 44,000 A/cm.sup.2 in a 1 Tesla field at 77K.
While the melt-texturing process has proven to be very effective in the fabrication of bulk YBa.sub.2 Cu.sub.3 O.sub.7-x having properties approaching those of thin film materials, it has substantial drawbacks. First, melt-texturing is essentially a crystal growth process in which the rate of material production is controlled by the velocity of the crystallization front. In the case of YBa.sub.2 Cu.sub.3 O.sub.7-x crystallizing out of its peritectic mixture, the crystallization rate is extremely sluggish. Even in extremely large thermal gradients (10.sup.7 K/m) growth rates of only 1.2 cm/hr have been achieved. A second problem, of particular importance to texturing thick film structures, is the fact that the melt-texturing process requires processing at temperatures above 1000.degree. C. in the presence of the extremely reactive peritectic liquid. This severely limits the choice of substrate materials that can be used without reacting with the superconductor. To date, only zirconia and magnesia have been used with any degree of success, and these ceramics are expensive and difficult to process.
It is therefore an object of the invention to provide an improved method of producing a high temperature superconductor (HTSC) material.
It is a further object of the invention to provide a novel method of producing an HTSC material at relatively low temperatures with very high rate of production.
It is another object of the invention to provide an improved method of producing HTSC structures on relatively inexpensive substrates.
It is yet a further object of the invention to provide a novel HTSC article of manufacture.
It is still an additional object of the invention to provide an improved method of manufacturing HTSC material and an article of manufacture having relatively intricate patterns formed directly from the process.
It is still another object of the invention to provide a novel HTSC article of manufacture of an intermediate phase state having minimal liquid involved in its manufacture.
Further objects and advantages of the present invention, together with the organization and manner of operation thereof; will become apparent from the following detailed description of the invention when taken in conjunction with the accompanying drawings, wherein like elements have like numerals throughout the drawings.